Such an injection device is known from document DE 299 07 881 U1, which discloses an injector device for automatically, in a first step injecting, the needle, and in a second step dispensing an injection dose, the injection dose being beforehand set by manual operation. In this device the injection operation is mainly effected by a pre-stressed spring which urges the injection mechanism toward a proximal end (side of a patient) of the device, when the dispensing of an injection dose is activated. In the first step (needle injection) the aforementioned spring acts on a threaded sleeve having an inner thread being in contact with the outer thread of the piston rod. The piston rod is coupled by a slipping clutch (holding means) with further sleeves that are fixedly connected with the container having the needle at its proximate end. In the first step, the spring urges via threaded element, piston rod, slipping clutch, and sleeves in axial direction on the container so that the needle is moved together with the container to a stop position. When this stop position is reached, the pre-stressed spring urges further on the threaded element and the piston rod in proximal direction, so that the slipping clutch having ratchet teeth on the piston rod and a corresponding ratchet arm on a further sleeve, is disengaged. This enables the piston rod to be axially displaced in order to press on the piston within the container and to dispense the beforehand set injection dose. The slipping clutch, namely the ratchet arm, is arranged on these sleeves being in contact with the container, so that the whole slipping clutch is axially moveable with respect to the housing of the injection device.
Furthermore, there is known a similar injection device from document EP 1 610 848 B1 having no needle expelling function and no holding means.
It is often to observe that a little bit of the medicament to be injected leaks from the needle when a patient is rotating a dose knob of the dose setting mechanism. Typically, there appears firstly a droplet on the needle point. When further rotating action is made on the dose knob this droplet grows further and in extreme examples the medicament drop falls down. Such phenomena are unintentional and are making the patient insecure, although these phenomena are not safety relevant in addition to the contamination of the environment. Such phenomena are due to manufacturing tolerances, material surface properties, undesirable inner frictions and plays within the injection mechanism, which leads to unwanted axial movements of the piston rod dispensing medicament from the cartridge.
Such phenomena could be avoided by extremely precise manufacturing and frictionless construction of the different components of the injection mechanism. This is very costly and it is very difficult to achieve such precise manufacturing, also bearing in mind, that the components are rather small.